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Jonathan Horton, MD, PhD

The visual system provides a supremely efficient means for the rapid assimilation of information from the environment to guide human behavior. Images are converted by the retina into action potentials, which are conduced along the optic pathway to the lateral geniculate body. Virtually the entire output of the lateral geniculate body is relayed to the primary visual cortex. Within the primary visual cortex, signals are processed by cells arranged within an elaborate system comprised of overlapping vertical columns and horizontal layers. Our first goal is to map the functional architecture of the primary visual cortex, to understand how groups of cells are organized in a modular fashion for information analysis. We are using autoradiography, axon tracing, cytochrome oxidase histochemistry, functional gene expression, and electrophysiology to accomplish this aim.

After initial processing the primary visual cortex, images are transferred to several dozen extrastriate visual areas, where perception takes place. Our second goal is to map the functional architecture, boundaries, layout, and topography of extrastriate visual cortex. At present, our attention is focused upon areas V2, V3, V4, and V5, which are located close to V1 in flat-mounted specimens of visual cortex.

Amblyopia is a disease of the cortex caused by visual deprivation. The most severe form occurs in a child who grows up with a dense unilateral cataract. Even after removal of the cataract, vision remains poor because connections serving the amblyopic eye are miswired within the brain. Our third goal is to delineate the critical periods for normal development and plasticity of the visual cortex, and to understand how projections within the cortex are disrupted by early visual deprivation and strabismus.

We use the macaque for most of our research, because it provides an excellent model for visual processing in the human brain. In addition, whenever possible, we conduct parallel anatomical studies in specimens of human visual cortex obtained post-mortem. This approach has allowed us to extend and confirm many of our findings, giving us confidence that our animal experiments are yielding valid insights into the function of the human visual cortex.

Current Projects

We are pursuing two major projects in the laboratory. The first examines temporal coding of information in the anesthetized macaque monkey. We are comparing the information content in S-potentials (retinal ganglion cell afferents) and principal cells of the lateral geniculate nucleus. Our goal is to learn how information content changes across the geniculate synapse, and how it differs in geniculate cells driven by a normal eye versus an amblyopic eye. The second project addresses the mechanism of visual suppression in strabismus. We are seeking to learn, in awake monkeys raised with strabismus, how images from the deviated eye are suppressed perceptually. This may yield new insights into the mechanism underlying spontaneous development of strabismus in children and provide insight into how binocular rivalry is mediated in normal subjects.